In recent years, in image forming methods utilizing an electrophotographic system, image formation, utilizing the digital system, has been playing an increasing role due to progress in digital technology. In the image forming method utilizing said digital system, an image comprised of minute dots such as a pixel of 1,200 dpi (the number of dots per inch) is fundamentally visualized. As a result, high imaging technology is demanded so that images comprised of minute dots are faithfully reproduced.
From the viewpoint of achieving such high quality images, a decrease in the diameter of toner particles has been investigated. In the formation of electrophotographic images, heretofore, a so-called pulverized toner has been mainly employed. Said pulverized toner has been prepared in such a manner that binder resins and pigments are blended, kneaded, and subsequently pulverized, and the resultant toner powder is classified through a classifying process. The toner prepared employing such production processes is limited in decreasing the toner particle diameter as well as in narrowing the particle size distribution. It has been difficult to prepare toner particles having the desired particle size distribution as well as the desired uniform particle shape. As a result, it has been difficult to enhance the quality of electrophotographic images prepared by employing such pulverized toner to the demanded degree.
Recently, as a means to decrease the diameter of toner particles, to narrow the particle size distribution, and to acquire uniform particles, a polymerization toner, prepared by employing a suspension polymerization method or an emulsion polymerization method, has received attention. Said polymerization toner is prepared in such a manner that a raw material monomer is uniformly dispersed in a water based medium and subsequently undergoes polymerization. Therefore, technology to achieve coalescence (aggregation and fusion) of resinous particles prepared by said suspension polymerization or said emulsion polymerization with colorant particles is in the development stage.
On the other hand, as a system to fix toner images formed on an image forming support such as a paper sheet, a heating roll fixing system is widely employed in which said image forming support, on which a toner image has been formed, is passed between a heating roller and a pressing roller so as to achieve fixing. However, said heating roll fixing system has problems in which images tend to be stained due to off-setting in which the fused toner adheres to said heating roller.
As a means to minimize off-setting, it has been known that releasing properties can be provided on said heating roller by applying silicone oil onto the surface of said heating roller in a fixing unit. Said means is useful because the kinds of employed toner are not limited.
However, said means has not been suitable for preparing office documents because silicone oil is applied onto transfer sheets to make it impossible to write on said sheets with writing means such as ballpoint pens. Further, said means causes problems in that components, contained in said silicone oil, become volatile due to heat on the heating roller surface and adhere to and stain the optical system, as well as charging electrodes whereby image problems result.
From the viewpoint of the foregoing, oil-free fixing techniques have been increasingly demanded in which said silicone oil is not supplied to the fixing unit (or the heating roller) or the amount of said silicone oil supplied to said fixing unit is markedly decreased.
Further, machines have become more common which are fitted with units for high speed operation, which produce the number of copies with response to print-on-demand, and are fitted with finishers such as an inner binding bookbinding mechanism. In these units, during bookbinding, bound paper sheets come into contact with each other, and problems occur in which toner on a paper sheet is transferred onto the white background of the paper sheet of the following page and is rubbed to result in staining.
In order to overcome such drawbacks, providing releasing properties to the toner itself has been widely carried out by incorporating releasing agents such as wax into said toner. Releasing agent particles are incorporated into the toner (the polymerization toner) which is prepared employing the aforesaid polymerization methods, and a method is known in which during the polymerization process, resinous particles and releasing agent particles are allowed to coalesce. However, it is extremely difficult to incorporate a sufficient amount of releasing agent into the interior of said coalesced particles (colored particles) of the toner prepared by said methods. Further, the content of said releasing argents among resultant coalesced particles fluctuates. As a result, the toner as a whole is not capable of exhibiting sufficient releasing properties (off-setting resistance).
In order to overcome these drawbacks, as disclosed in Japanese Patent in Publication Open to Public Inspection No 8-41468, compounds employed for releasing agents are improved and techniques to increase the addition amount into toner are investigated. However, due to the structural cause in that toner is comprised of coalesced particles of resinous particles and releasing agent particles, releasing agents are released from coalesced particles, and the resultant releasing agents cause problems whereby said releasing agents degrade developers, decrease toner fluidity, and generate photoreceptor filming.
Further, Japanese Patent Publication Open to Public Inspection Nos. 3-296067, 10-161338, and 5-88409 disclose toners which are comprised of a domain-matrix structure such that a releasing agent phase in the toner particle structure is subjected to phase separation in the binding resin phase. However, these patent publications do not mention any fluctuation of the added amount of releasing agents in toner particles and do not disclose any suggestion to control said fluctuation. Further, even though toners disclosed in these are comprised of a structure in which releasing agents are subjected to phase separation, the resultant releasing agent phase exists in the center of the toner particle as an aggregate, whereby it is difficult to effectively bleed the releasing agents to the exterior of the toner particle. As a result, it is inevitable to increase the amount of releasing agents incorporated into toner particles. Further, it is difficult to bleed an optimal amount of the releasing agents to the exterior of the particle during image fixing, due to the fact that said releasing agents are required to migrate to the exterior of the particle. As a result, it has been extremely difficult to sufficiently minimize off-setting.